Spring---循环依赖-@Autowired

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/**
     *  【循环依赖---@Autowired】
     *
     *      BeanPostProcessor ： org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor
     *
     *                              org.springframework.beans.factory.config.BeanPostProcessor implementation that autowires annotated fields, setter methods, and arbitrary config methods.
     *                              BeanPostProcessor实现，用来自动织入 注解字段...;
     *                              Such members to be injected are detected through annotations: by default, Spring's @Autowired and @Value annotations.
     *                              一些成员将被检测到：Spring的@Autowired、@Value；
     *
     *      public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
     *
     *          public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {}
     *      }
     *
     *      1、
     *      DefaultSingletonBeanRegistry#getSingleton(String beanName){
     *          Object singletonObject = this.singletonObjects.get(beanName);
     *          if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
     *              synchronized (this.singletonObjects) {
     *                  singletonObject = this.earlySingletonObjects.get(beanName);
     *                  if (singletonObject == null && allowEarlyReference) {
     *                      ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
     *                      if (singletonFactory != null) {
     *                          singletonObject = singletonFactory.getObject();            === 2、
     *                          this.earlySingletonObjects.put(beanName, singletonObject);
     *                          this.singletonFactories.remove(beanName);
     *                      }
     *                 }
     *             }
     *        }
     *        return singletonObject;
     *      }
     *
     *      2、
     *      AbstractAutowireCapableBeanFactory#getEarlyBeanReference{
     *          Object exposedObject = bean;
     *          if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
     *              for (BeanPostProcessor bp : getBeanPostProcessors()) {
     *                  if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
     *                      SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
     *                      exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
     *                  }
     *              }
     *          }
     *          return exposedObject;
     *      }
     *
     *      3、
     *      DefaultSingletonBeanRegistry#getSingleton(String, ObjectFactory){
     *          ...
     *          singletonObject = singletonFactory.getObject();
     *          ...
     *          if (newSingleton) {
     *              addSingleton(beanName, singletonObject);  === 4、
     *          }
     *          ...
     *      }
     *
     *      4、DefaultSingletonBeanRegistry#addSingleton{
     *          synchronized (this.singletonObjects) {
     *              this.singletonObjects.put(beanName, singletonObject);
     *              this.singletonFactories.remove(beanName);
     *              this.earlySingletonObjects.remove(beanName);
     *              this.registeredSingletons.add(beanName);
     *          }
     *      }
     *
     *      5、
     *      DefaultSingletonBeanRegistry#addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory){
     *          synchronized (this.singletonObjects) {
     *              if (!this.singletonObjects.containsKey(beanName)) {
     *                  this.singletonFactories.put(beanName, singletonFactory);
     *                  this.earlySingletonObjects.remove(beanName);
     *                  this.registeredSingletons.add(beanName);
     *              }
     *          }
     *      }
     *
     *
     *      实现思路：
     *
     *      AbstractBeanFactory#getBean
     *      ->
     *      AbstractBeanFactory#doGetBean{
     *          ...
     *          Object sharedInstance = getSingleton(beanName);   === 1、
     *          if (sharedInstance != null && args == null) {
     *              ...
     *          }
     *          else{
     *              ...
     *              if (mbd.isSingleton()) {
     *                  sharedInstance = getSingleton(beanName, () -> {   === 3、
     *                      return createBean(beanName, mbd, args);
     *                  }
     *                  ...
     *              }
     *              ...
     *          }
     *          ...
     *      }
     *      ->
     *      DefaultSingletonBeanRegistry#getSingleton(String, ObjectFactory) -> singletonFactory.getObject()
     *                                                                    AbstractBeanFactory#createBean <-
     *      ->
     *      AbstractAutowireCapableBeanFactory#createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args){
     *          ...
     *          Object beanInstance = doCreateBean(beanName, mbdToUse, args);
     *          ...
     *      }
     *      ->
     *      AbstractAutowireCapableBeanFactory#doCreateBean(java.lang.String, RootBeanDefinition, java.lang.Object[]){
     *          ...
     *          instanceWrapper = createBeanInstance(beanName, mbd, args);
     *          ...
     *          boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName));
     *          if (earlySingletonExposure) {
     *              addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));   === 5、
     *          }
     *          ...
     *          populateBean(beanName, mbd, instanceWrapper);
     *          ...
     *      }
     *      ->
     *      AbstractAutowireCapableBeanFactory#populateBean{
     *          ...
     *          if (hasInstAwareBpps) {
     *              for (BeanPostProcessor bp : getBeanPostProcessors()) {
     *                  ...
     *                  PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
     *                  ...
     *              }
     *          }
     *          ...
     *      }
     *      ->
     *      AutowiredAnnotationBeanPostProcessor#postProcessProperties{
     *          ...
     *          metadata.inject(bean, beanName, pvs);
     *          ...
     *      }
     *      ->
     *      InjectionMetadata#inject{
     *          ...
     *          element.inject(target, beanName, pvs);
     *          ...
     *      }
     *      ->
     *      AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject{
     *          ...
     *          value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
     *          ...
     *      }
     *      ->
     *      DefaultListableBeanFactory#resolveDependency{
     *          ...
     *          result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
     *          ...
     *      }
     *      ->
     *      DefaultListableBeanFactory#doResolveDependency{
     *          ...
     *          instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
     *          ...
     *      }
     *      DependencyDescriptor#resolveCandidate{
     *          return beanFactory.getBean(beanName);
     *      }
     *      ->
     *      递归
     *
     *
     */